Deimos also passes nearly twice through all its phases whilst it is above the horizon, viz. during about 60 hours, and may be seen twice full and twice new in that time.
Eclipses of Deimos by the planet and occultations of it by the other satellite are very frequent. Being so small, it can never cause an eclipse of the sun, but it transits the sun as a dark spot about one hundred and twenty times during the Martian year.
This is really a very inadequate list of the phenomena connected with the satellites, but it will be seen that the number is enormous compared with the few eclipses of the sun or moon seen on the earth during the course of one year. Certainly Mars is an astronomer's world!
Merna heard my statements respecting these movements and phenomena as I explained them to my two friends; and when I had finished, he remarked, "You seem to be fairly well posted in these matters, sir?"
"Yes," I said; "thanks to our astronomers, both professional and amateur, all these things have been very carefully calculated; and, with the exception of a few doubtful points, we probably know nearly as much about them as the Martians themselves do."
M'Allister then turned to me and said, "Professor, you told us that the two satellites of Mars revolved round the planet in a certain time, but in each case you afterwards said they appeared to take a much longer time to do so. I'm rather puzzled to understand how that can be."
"It's really a simple matter, M'Allister," I answered, "and I think I can make it clear to you. While the satellite is making one revolution round the planet the latter is turning on its axis in the same direction as the satellite is moving, following it up in fact; and you will I think understand that in these circumstances the people on that part of the planet where the moon is visible must necessarily keep it in view for a longer period than would be the case if the planet were not revolving in the same direction.
"You have been used to being on board a ship; so suppose your vessel was steaming twelve miles an hour and there was another vessel at anchor just twelve miles ahead of you, you would reach it in just one hour, would you not?"
"Yes, certainly I should," replied M'Allister.
"Now," I continued, "suppose that the other vessel, instead of being at rest, was moving away from you at the rate of six miles an hour; after you had steamed one hour it would still be six miles ahead of you, and it would take you exactly another hour to catch it up. So you would be just double the time reaching it when moving as compared with the time required to do so when it was at anchor. This is very similar to the cases of the satellites of Mars, and much the same thing happens in regard to Mars and the earth. If they are opposite to each other at a certain point, Mars will have taken much more than one revolution round its orbit before they will be opposite to each other again, because they are both moving in the same direction. Do you see it now?" I asked.